The present invention is related generally to the field of heat transfer and more specifically to the field of thermal contact resistance during heat transfer.
As modern electronic devices decrease in size and increase in speed, power density rises dramatically. As power density rises, so does the heat produced by these devices. Traditionally, designers have relied on airflow over heat generating parts for cooling. In vacuum tube systems, large enclosures with numerous ventilation openings allowed the heated air to escape the enclosure. Later systems, such as many personal computers, included fans within an enclosure to draw cool air from outside the enclosure force it over the heat generating devices, and push the warmed air out through ventilation openings.
Many large modern electronic systems require more cooling than simple airflow, whether forced or not, is capable of providing. Some large computers use a network of cold plates plumbed together with water lines to use water as a thermal liquid carrying the heat away from the heat generating devices. Such systems are very expensive and since each cold plate must be plumbed, repair of these systems becomes very complicated. For example, in replacing a circuit board that has one or more heat generating devices cooled by cold plates, each individual cold plate must either be removed from the device or the plumbing must be disconnected from the overall system before the circuit board can be removed. Use of liquid cooling may make hot swapping boards impossible. The connectors for the liquid would have to be opened in areas of the system including live voltages situated such that even a small spill would be very likely to result in a short circuit with resulting damage to the system, and possibly the user. This makes maintenance of such systems more time-consuming and therefore more costly, particularly when including the risk of leaks.
Modern computer servers often comprise a number of individual server modules plugged into racks that supply power to the modules and interconnect the modules to memory, storage, and each other. Such racks present difficult thermal problems, since a large number of heat generating devices are often placed within relatively small server modules that are then placed together tightly in the rack. Ventilation may be constrained by the rack and it""s necessary components, and by the fact that often users will want to place servers close together to reduce floor space required in their computer rooms. Liquid cooling becomes very attractive in situations like this, since liquids are capable of carrying a much greater thermal load than air. However, liquid cooling each server module would require plumbing each server module which, while possible, would eliminate much of the benefit of having readily replaceable server modules.
Another problem encountered in systems allowing easy replacement of boards is the issue of tolerance between the board and the rack. Often even boards of identical design will have small manufacturing tolerance differences in their dimensions. Thermal transfer systems relying on contact between a board and an external heat sink may require a greater dimensional tolerance from board to board than may be available with standard thermal grease or elastomeric conductors. Thermal grease only provides a few mils of tolerance. For many applications this is insufficient. This problem only gets worse when systems do not have dedicated slots for each board design, but allow differing boards to be placed in any given location. These different boards may include different thermal transfer needs. They may include heat sinks at different locations on the board and may generate different amounts of power. These problems make it difficult to design a simple heat transfer system for a large system that still allows for flexibility of system configurations.
A variable gap thermal interface is coupled with a cold or hot plate, forming a low thermal resistance connection between an electronic device module containing at least one heat generating electronic device and a rack or other structure. The variable gap thermal interface and the cold or hot plate are provided in a configuration to allow quick-disconnect of the electronic device module from the rack, allowing for a wide dimensional tolerance between the module and the rack while maintaining a reliable thermal connection. An embodiment including a plurality of server modules within a server rack in conformance with the present invention, allows the replacement of server modules while powered without any disconnection or reconnection of hoses to cold plates used in cooling the server modules, thus greatly reducing the probability of leaks and resulting damage to the system.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.